Scanning Kelvin Force Microscopy (KFM)  is a technique based on Atomic Force Microscopy (AFM)for simultaneous recording of topography and surface potential. KFM applied for GaN epi-layers  revealed that surface potential in n-type material is related to the threading dislocations due to their charge states and the contrast was dependent on type and level of doping i.e. position of the Fermi level .
We report on the fluctuations in the surface potential, that appear during illumination close to the energy gap of GaN. The investigated samples were high quality undoped GaN layers grown on sapphire by Metal-Organic Vapour Phase Epitaxy.The AFM allows to find that the density of dislocations is of the order 108 cm-2 and distinguish between edge and screw type dislocations. The surface potential measured in dark is almost flat and featureless. Two types of potential fluctuations can be observed. The first one, "short range"(~200nm) shows a clear correlation with screw type dislocations, i.e. the potential peak usually surrounds a dislocation pit. This is explained as an effect of negative charging of dislocations. However in some samples the potential peaks are more numerous then pits observed on the surface. This can be understood as an image of dislocations below the surface of the samle.
The "long range"(~5 μm) fluctuations are visible in the 25x25 μm images and can be assigned to heterogenous distribution of impurities due to strain or mosaic structure of the epilayer.
The observed fluctuations of the potential are strongly dependent on the intensity of the illumination. Very intense light flattens the potential distribution and removes observed contrast.
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